The present invention relates to elastomer compositions comprising silica as reinforcing filler, said compositions having improved hysteretic properties in the vulcanized state.
Since economizing on fuel and protection of the environment have become priorities, it is desirable to produce mixtures having the lowest possible hysteresis, in order to be able to use them as semi-finished products which enter into the make-up of tire casings, semi-finished products such as undercoats, binding gums between rubbers of different types, or for calendering metallic or textile reinforcements, sidewall gums or treads, and to obtain tires with improved properties, said tires having a reduced resistance to rolling in particular.
To achieve this objective, numerous solutions have been proposed consisting, in particular, in modifying tile nature of the diene polymers and copolymers at the end of polymerization by means of coupling agents, starring agents or functionalizating agents. The great majority of these solutions are essentially concentrated on the use of polymers modified with carbon black as reinforcing filler, with a view to obtaining a good interaction between the modified polymer and the carbon black, since the use of white reinforcing agents, notably silica, has proved inappropriate owing to the low value of some of its properties, and, as a result, of certain properties of the tires using these compositions.
As an illustrative example from the prior art, mention may be made of U.S. Pat. No. 4,677,165, which describes the reaction of living diene polymers functionalized by means of a benzophenone derivative to obtain polymers having improved properties in compositions containing carbon black as reinforcing filler. EP-A-0,451,604 describes, as functionalizing agent, a compound having an amino function which permits an improved interaction between the modified polymers and carbon black. With the same objective, U.S. Pat. No. 4,647,625 describes the functionalization of elastomers by reaction of a living polymer with N-methylpyrrolidine. EP-A-0,590,491 and EP-A-0,593,049 describe polymers having a tertiary amino function at the end of the chain and also permitting better interaction with carbon black.
Certain solutions have also been proposed concerning the use of silica as reinforcing filler in the compositions designed to constitute treads of tires. Functionalized polymers comprising alkoxysilane functions at the end of the polymer chains, as described in EP-A-0,299,074 and EP-A-0,447,066, have been proposed For this purpose. It is only these functionalized polymers that have been described in the prior art as being effective in reducing the hysteresis and improving the resistance to abrasion, but the industrial manufacture of these polymers poses problems due to macrostructure development during the recovery stages such as stripping or drying. To attempt to solve these problems, the use of polymers functionalized with non-hydrolyzable alkoxysilane functions, as described in U.S. Pat. No. 5,066,721 have been proposed, but their effectiveness in a mixture with silica is reduced. The preparation of diene polymers having an amino function is known to persons skilled in the art, e.g. from U.S. Pat. No. 4,894,409, which describes the preparation of polymers functionalized with aromatic amines. The polymers having an amine function are not very effective when used with silica, as mentioned in EP-A-0,661,298.
This interest in silica-reinforced compositions was revived with the publication of EP-A-0,501,227 which discloses a rubber composition vulcanizable with sulfur, obtained by thermomechanical working of a copolymer of a conjugated diene and a vinylaromatic compound, prepared by solution polymerization, with 30 to 150 parts by weight per 100 parts by weight of elastomer of a particular precipitated silica, which represents an excellent compromise between several contradictory properties, and which for the first time permits the commercialization of tires possessing a silica-filled tread showing the excellent compromise that is required.
Mention may also be made of European Patent document EP-A-819,731, which discloses a silica-reinforced composition essentially comprising a diene elastomer consisting of a copolymer having amino functions and a silica/elastomer linking agent of the sulfurated silane type. Along its chain, this copolymer has amino groups originating from nonaliphatic or cycloaliphatic amines and an aminated vinylaromatic monomer.
It will be noted that the problem that is supposed to be solved in this last document does not relate to obtaining improved hysteretic properties.
The present invention relates to a new rubber composition containing silica or a mixture of silica and carbon black as reinforcing filler, said composition having satisfactory properties when used in the crude state and improved properties in the vulcanized state, particularly an excellent hysteresis and excellent reinforcement.
The invention also relates to treads of tires comprising at least one composition in accordance with the invention, as well as to treads obtained by vulcanization of these treads.
The invention also relates to casings of tires comprising at least one composition according to the invention, as well as to tire casings obtained by vulcanization of these casings, at least one composition of the invention being used, e.g., in the tread.
The rubber composition vulcanizable with sulfur, according to the invention, is characterized in that it comprises:
a) at least one polymer selected from the group consisting of diene polymers, olefin/monomeric diene copolymers and halogenated isoolefin/para-alkylstyrene copolymers;
b) silica as filler;
c) at least one agent promoting binding between silica and polymer, said agent having the formula
Z1xe2x80x94R1xe2x80x94Snxe2x80x94R2xe2x80x94Z2xe2x80x83xe2x80x83(I)
wherein
n is an integer between 2 and 8,
R1 and R2, which may be the same or different, are each selected from among substituted or unsubstituted alkylene groups having 1 to 18 carbon atoms and substituted or unsubstituted arylene groups having 6 to 12 carbon atoms,
Z1 and Z2, which may be the same or different, each represents a group
R3 
xe2x80x94Sixe2x80x94R4
R5 
where R3, R4 and R5, which may be the same or different, are each selected from among alkyl groups having 1 to 4 carbon atoms, phenyl groups, alkoxy groups having 1 to 8 carbon atoms, and cycloalkoxy groups having 5 to 8 carbon atoms, with the proviso that at least one of R3, R4 and R5 is an alkoxy or cycloalkoxy; and
d) at least one guanidine substituted by at least two groups, which may be the same or different, and selected from among alkyl, aryl or aralkyl groups;
and wherein the rubber composition has at least one of the following characteristics:
It comprises at least one diene polymer which has at least one terminal amino group of an aliphatic or cycloaliphatic amine which is bonded thereto at the end of the chain, the polymer then being devoid of alkoxysilane and silanol group;
The composition comprises at least one free aliphatic or cycloaliphatic amine.
In the description that follows, the term xe2x80x9cpolymerxe2x80x9d covers both homopolymers and copolymers; the term xe2x80x9ccopolymerxe2x80x9d referring to polymers obtained from two or more than two monomers, e.g. terpolymers.
Understood by a diene polymer is any homopolymer obtained by polymerization of a conjugated diene monomer having 4 to 12 carbon atoms, any copolymer obtained by copolymerization of one or more conjugated diene with each other or with one or more vinylaromatic compounds having 8 to 20 carbon atoms. As conjugated diene, mention may be made of e.g. 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-di(C1-C5-alkyl)-1,3-butadiene such as, e.g., 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1,3-butadiene, phenyl-1,3-butadiene, an aryl-1,3-butadiene, 1,3-pentadiene, and 2,4-hexadiene.
Suitable vinylaromatic compounds are, in particular, styrene, ortho-, meta- and paramethylstyrene, the commercial xe2x80x9cvinyl-toluenexe2x80x9d mixture, p-tert.-butylstyrene, the methoxystyrenes, chlorostyrenes, vinylmesitylene, divinylbenzene, and vinylnaphthalene.
The copolymers may contain, e.g., between 99% and 20% by weight of diene units and from 1% to 80% by weight of vinylaromatic units. The polymers may have any microstructure, which is a function of the polymerization conditions used, notably of the presence or absence of a modifying agent and/or randomizing agent and of the amounts of modifying and/or randomizing agent used.
Polybutadienes are preferred, particularly those having a 1,2-linkage content of between 1% and 80%, as well as polyisoprenes, and copolymers of butadiene and styrene in particular, those having a styrene content of between 5 and 50% by weight, and more particularly between 20% and 40% by weight, a 1,2-linkage content of the butadiene part of between 4% and 65%, and a trans-1,4 linkage content of between 30% and 80%. Suitable butadiene-styrene-isoprene copolymers are those having a styrene content of between 5 and 50% by weight, more particularly between 10% and 40%, an isoprene content of between 15% and 60% by weight, more particularly between 20% and 50% by weight, and a butadiene content of between 5% and 50% by weight, more particularly between 20% and 40% by weight, wherein the butadiene part has a 1,2-linkage content of between 4% and 85% and a trans-1,4 linkage content of between 6% and 80%, and the isoprene part has a 1,2-linkage content plus 3,4-linkage content of between 5% and 70% and a trans-1,4-linkage content of between 10% and 50%.
When the diene polymer has an amino terminal group, it is then devoid of an alkoxysilane group and silanol group and may be a homopolymer or copolymer obtained from the above-mentioned monomers, wherein said polymer can be obtained by initiation or functionalization.
In the case where the terminal amino group is introduced by initiation, the polymerization is advantageously carried out with an initiator of the lithium amide type, as described in the literature, e.g. in the work of T. C. Cheng, xe2x80x9cAnionic Polymerization,xe2x80x9d published by the American Chemical Society, Washington, p. 513, 1981, whose content is incorporated herein. This lithium amide, has the formula 
wherein R1 and R2, independently of one another, are an alkyl group, or together form a cycloalkyl, the number of carbon atoms of the compound formed by R1 and R2 preferably being 2 to 15.
As a preferred example, R1 and R2 may each be an ethyl or butyl group, or together form a cycloalkyl radical having 4 to 6 carbon atoms.
When necessary, the initiator of formula (II) is prepared in the presence of a polar agent such as tetrahydrofuran, by reacting the corresponding amine R1xe2x80x94NHxe2x80x94R2, such as hexamethyleneimine, with an alkyllithium, such as n-butyllithium.
In the case where the amino end group is introduced by functionalization at the end of polymerization, the functionalization procedures described in, e.g., EP-A-451,604, U.S. Pat. No. 4,647,625 or in Kenji Ueda et al., xe2x80x9cSynthesis of polymers with amino end groups. 3. Reactions of anionic living polymers with xcex1-halo-xcfx89-aminoalkanes with a protected amino functionality,xe2x80x9d Macromolecules 23: 939-945 (1990), may be used.
The polymerization may be carried out by a continuous or a discontinuous process. The polymerization is generally carried out at a temperature between 20xc2x0 C. and 120xc2x0 C., preferably between 30xc2x0 C. and 90xc2x0 C.
The olefin/diene copolymers can be, notably, EPDM""s (ethylene-propylene-diene monomer), such as ethylene-propylene-1,4-hexadiene copolymers, ethylene-propylene-ethylidenenorbomene copolymers, ethylene-propylene-dicyclopentadiene copolymers, butyl rubbers, particularly isobutylene-halogenated isoprene copolymers, as the case may be. The isoolefin/halogenated para-alkylstyrene copolymers can be, e.g. the isobutylene-/halogenated para-methylstyrene rubber marketed by Exxon under the trade name EXXPRO.
The polymers may be, e.g., block polymers, statistical polymers, sequenced polymers, microsequenced polymers, and can be prepared by dispersion polymerization, solution polymerization, bulk polymerization, or in a gaseous phase.
The composition according to the invention may comprise a mixture of two or more polymers defined above.
The free aliphatic or cycloaliphatic amine may be a primary, secondary or tertiary amine. By way of nonlimiting examples, mention may be made, e.g., of the following amines: butyl, dibutyl, tributylamines, pentyl, dipentyl, tripentylamines, hexyl, dihexyl, trihexylamines, cyclohexyl, dicyclohexyl, tricyclohexyl amines, benzyl, dibenzyl, tribenzylamines, octyl, dioctyl, trioctylamines, decyl, didecylamines, dodecyl, didodecylamines, pyrrolidine and alkyl pyrrolidines, piperidine and alkylpiperidines, hexamethyleneimine and alkylhexamethyleneimines, wherein said amines may be used alone or in a mixture with one another.
Preferably, the free amine content or the mixture of free amines ranges from 0.5 to 4% by weight calculated on the total weight of silicas, said content advantageously being between 1 and 3% by weight.
The substituted guanidine may be, e.g., triphenylguanidine, diphenylguanidine, or di-o-tolylguanidine. The composition may comprise a mixture of two or more substituted guanidines.
The content of substituted guanidine or mixture of substituted guanidines is 0.5 to 4% by weight calculated on the silica or total of silicas, said content advantageously being 1 to 3% by weight.
The silica used as filler may be any silica known to persons skilled in the art having a BET surface of less than or equal to 450 m2/g, a specific surface area CTAB of less than or equal to 450 m2/g, even though this improvement is more pronounced with a highly dispersible precipitated silica. Understood by the term xe2x80x9chighly dispersible silicaxe2x80x9d is any silica capable of deagglomeration and dispersion in a very large amount of polymeric matrix, observable on fine sections by electron microscopy or optical microscopy. Nonlimiting examples of such preferred, highly dispersible silicas are those obtained according to the processes described in EP-A-0,157,703 and EP-A-0,520,862, or the Zeosil silica 1165 MP of Rhxc3x4ne-Poulenc, the Perksasil silica KS 430 of Akzo, silica Hi-Sil 2000 of PPG, and silicas Zeopol 8741 and Zeopol 8745 of Huber Co. According to the invention, the beneficial effect is obtained regardless of the physical state in which the silica is present, i.e whether it is in powder form, micropearl form, granulated form, spherical form, and regardless of what the specific surface area of the silica is. It is also possible to use blends of different silicas and use other white fillers, such as chalk, kaolin or alumina.
In combination with silica, it is possible, if necessary, to use carbon black in the reinforcing filler, notably all carbon blacks that are commercially available or conventionally used in tires, particularly in treads.
The filler comprises one or more silicas or it contains at least 40% by weightxe2x80x94and advantageously at least 50% by weightxe2x80x94a silica or mixture of silicas.
The reinforcing filler, which may thus comprise silica without carbon black or both silica and carbon black, is present in the composition in an amount which may range, e.g., from 30 to 100 parts by weight of the polymer or mixture of polymers.
By way of example of an agent of formula (I) which promotes linking between the silica or combination of silicas and the diene polymer or mixture of diene polymers, mention may be made of the 2,2xe2x80x2-bis(trimethoxysilylethyl) polysulfides, the 3,3xe2x80x2-bis(trimethoxysilylpropyl) polysulfides, the 3,3xe2x80x2-bis(triethoxysilylpropyl) polysulfides, the 2,2xe2x80x2-bis(triethoxysilylpropyl) polysulfides, the 2,2xe2x80x2-bis(tripropoxysilylethyl) polysulfides, the 2,2xe2x80x2-bis(tri-sec-butoxysilylethyl) polysulfides, the 3,3xe2x80x2-bis(tri-tert-butoxysilylethyl) polysulfides, the 3,3xe2x80x2-bis(triethoxysilylethyltolylene) polysulfides, the 3,3xe2x80x2-bis(trimethoxy-silylethyltolylene) polysulfides, the 3,3xe2x80x2-bis(triisopropoxysilylpropyl) polysulfides, the 3,3xe2x80x2-bis(trioctoxysilylpropyl) polysulfides, the 2,2xe2x80x2-bis(2xe2x80x2-ethylhexoxysilylethyl) polysulfides, the 2,2xe2x80x2-bis(dimethoxy-ethoxysilylethyl) polysulfides, the 3,3xe2x80x2-bis(methoxyethoxypropoxysilylpropyl) polysulfides, the 3,3xe2x80x2-bis(methoxydimethylsilylpropyl) polysulfides, the 3,3xe2x80x2-bis(cyclohexoxydimethylsilylpropyl) polysulfides, the 4,4xe2x80x2-bis(trimethoxy-silylbutyl) polysulfides, the 3,3xe2x80x2-bis(trimethoxysilyl-3-methylpropyl) polysulfides, the 3,3xe2x80x2-bis(tripropoxysilyl-3-methylpropyl) polysulfides, the 3,3xe2x80x2-bis(trimethoxymethylsilyl-3-ethylpropyl) polysulfides, the 3,3xe2x80x2-bis(trimethoxy-silyl-2-methylpropyl) polysulfides, the 3,3xe2x80x2-bis(dimethoxyphenylsilyl-2-methylpropyl) polysulfides, the 3,3xe2x80x2-bis(trimethoxysilylcyclohexyl) polysulfides, the 12,12xe2x80x2-bis(trimethoxysilyldodecyl) polysulfides, the 12,12xe2x80x2-bis(triethoxysilyl-dodecyl) polysulfides, the 18,18xe2x80x2-bis(trimethoxysilyloctadecyl) polysulfides, the 18,18xe2x80x2-bis(methoxydimethylsilyloctadecyl) polysulfides, the 2,2xe2x80x2-bis(trimethoxysilyl-2-methylethyl) polysulfides, the 2,2xe2x80x2-bis(triethoxysilyl-2-methylethyl) polysulfides, the 2,2xe2x80x2-bis(tripropoxysilyl-2-methylethyl) polysulfides, and the 2,2xe2x80x2-bis(trioctoxysilyl-2-methylethyl) polysulfides. Such a linking agent is, e.g., that sold by Degussa under the trade designation Si69 whose average formula is bis(3-triethoxysilylorioyl) tetrasulfide. These polysulfides can be used alone or in a mixture with each other.
By preference, n in the linking agent of formula (I) is greater than 2, or if there are several such agents, at least 80% of n is greater than 2 for the total of these compounds.
The content of the agent of formula (I) or mixture of such agents is preferably at least 4% of the weight of silica or of the mixture of silicas used as filler.
The compositions according to the invention can also contain other constituents and additives conventionally used in rubber mixtures, such as plasticizers, pigments, antioxidants, sulfur, vulcanization accelerants, extender oils, one or more linking agents of formulas other than (I) and/or one or more silica coating agents such as polyols, alkoxysilanes, in addition to the agent of formula (I). Preferably only one or more agents of formula (I) are used as silica coating agents, or if other linking agents are present, the weight of the linking agent or agents of formula (I) is greater than 50% of the weight of the total of linking agents.
The invention is illustrated in more detail by the following examples, which in no way limit the scope of the invention.
In the examples, the properties of the fillers, particularly of silica, and the properties of the compositions are evaluated as follows:
Mooney viscosity: ML(1+4) at 100xc2x0 C. measured according to ASTP D-1646; it will be referred to below as xe2x80x9cML(1+4)100xc2x0.xe2x80x9d
Elongation modulus at 300% (EM 300), 100% (EM 100) and 10% (EM 10), determined according to the standard ISO 37.
Scott tear index measured at 20xc2x0 C. (hereinafter abbreviated as Scott20xc2x0), the rupture force (Fr) is expressed in MPa and the elongation at rupture (Er) is expressed in %.
Hysteretic losses: Measured by rebound at 60xc2x0 C., the deformation is of the order of 40%; these losses are referred to below as xe2x80x9cHLxe2x80x9d, the word deformation being abbreviated to xe2x80x9cdef.xe2x80x9d
Dynamic properties under shear, according to ASTM D2231-71 (reapproved in 1977), measurements carried out as a function of the deformation at 23xc2x0 C. and 10 Hz. The nonlinearity, expressed in MPa, which is the difference of shear modulus between 0.15% and 50% of the peak-to-peak deformation, is referred to below as xe2x80x9cDeltaG*.xe2x80x9d The hysteresis is expressed by measurement of tgxcex4 and Gxe2x80x3 at 7% deformation.
In all the examples which follow, the values relating to the various components of the compositionsxe2x80x94unless otherwise indicatedxe2x80x94are given as parts by weight per 100 parts by weight of elastomer or mixture of elastomers (pce); the styrene contents of the polymers are expressed in % by mass, and the content of 1,2-vinyl chains of the polymers is expressed in % relative to the butadiene units incorporated.